Generally, lead screws are designed to engage with a nut or some other device and locked in place. Typical lead screws are designed to screw in one or two times and are then retained in a given location. These lead screws, preferably, are held in place once inserted into a device, such as by self-locking. Designs as are set forth herein have been created to prevent the screws from moving once fitted into a device so that self-loosening (i.e. back-drive) is eliminated. Examples of screw and nut combinations with varying thread flank profiles to prevent a screw from rotating loose once secured are disclosed in U.S. Pat. Nos. 2,036,604; 2,581,690; 3,323,402; 4,540,321; and 5,190,424 which are expressly incorporated herein by reference for all purpose. These disclosures teach arcuate flank surfaces to prevent displacement of threads or loosening from vibration after the screws have been secured. These disclosures do not teach how to incorporate arcuate flank surfaces into screws which are repetitively rotated within a mating threaded component, such as in an actuator assembly, or into a brake assembly.
Customarily, in traditional parking brake systems, a vehicle driver manually engages an integrated parking brake (IPB) or a drum-in-hat (DIH) parking brake via a hand or foot operated lever or pedal. In these traditional parking brake systems, the driver manually controls the brake application force applied to the parking brake through the lever or pedal. In electromechanical parking brake (EPB) systems, controls software is required to control the brake application force. In order to determine and apply a sufficient parking brake force, the controls software typically takes into consideration variation in electrical and mechanical components of the electromechanical parking brake system. One of these components is a linear stage assembly, which may be part of an actuator assembly and may convert a rotational force (i.e. torque) into a linear force. A motor, such as a motor in direct or indirect communication (i.e., motor gear unit), may drive a component, such as a screw or nut, linearly and/or axially so as to convert the rotational force into the linear force. The motor size is dependent on the frictional force between the screw and the nut of the linear stage assembly which must be overcome to drive the screw or the nut with respect to the other. The larger the variation in the electromechanical parking brake system, including variation in the linear stage assembly, the larger a motor is necessary to actuate a screw or nut within a linear stage assembly to overcome the frictional force. One of the variations in an actuator assembly is variation in the frictional force between a nut and screw of the linear stage assembly as the screw and nut axially and/or linearly move in relation to one another. The variation in the frictional force can be due to wear at contact surfaces between the nut and the screw or variability in the surface profiles. Additionally, a larger overall variation of the electromechanical parking brake system leads to an increase in the cost and the mass of the electromechanical parking brake system.
A linear stage assembly in a brake assembly generally comprises a screw (i.e., spindle) engaged with a nut. Both the screw and the nut include threads which are substantially flat (i.e., linear) on both sides in a cross-sectional view of the thread. When a brake apply force in the form of brake apply torque is applied to the linear stage assembly, a flat surface of the screw thread engages with a flat surface of the nut thread along a substantially linear contact interface. The substantially linear contact interface provides for variation in the friction which can result in brake apply torque variation equal to or greater than 7%. The variation in friction can be a result of manufacturing variability, such as the angle of a flat flank surface on a nut with respect to the nut axis may not be equal to the angle of a flat flank surface on a screw with respect to the screw axis; which may result in the flat flank surface of the nut being skewed (i.e., not parallel) from the flat flank surface of the screw when the screw is mated with the nut. Dimensional variation may then result as contact of the two flat flanks will occur either closer (i.e., at the thread root) or further (i.e., at the thread crest) from the screw and nut axes.
What is needed is a way to reduce component variation in a brake assembly, including mechanical, electrical or software variation. What is needed is a linear stage assembly, such as that of an actuator assembly, which may reduce variation in a brake apply force. What is needed is a linear stage assembly, such as that of an actuator assembly, which may sustain a brake application force (i.e., clamp force) after power is removed from the actuator assembly. What is needed is a linear stage assembly, such as that of an actuator assembly, with reduced variation in friction between a screw and nut during brake application (i.e., parking brake application). What is needed is an actuator assembly in which a constant and repeatable frictional force is required during engagement of the screw with the nut. What is needed is a linear stage assembly with sufficient durability to be utilized in a brake assembly. What is needed is a linear stage assembly that can repeatedly be used tens or even hundreds of thousands of times while preventing back drive and having a low load variation from actuation to actuation.